This invention is generally related to imaging systems and more particularly to gain and error correction in image sensors.
The image sensor is at the heart of all modern electronic imaging systems, including such consumer products as video cameras, copiers, scanners, and, more recently, digital cameras. In an imaging system, the image sensor is exposed to an object or changing scene to electronically capture image frames. These frames can then be processed using a variety of analog and digital image processing techniques to yield video or still images of the object or changing scene.
Modern electronic image sensors are built using different semiconductor technologies, including charge coupled device (CCD) and metal oxide semiconductor (MOS) fabrication processes. Different examples of image sensors are discussed in "An 800K-Pixel Color CMOS Sensor for Consumer Still Cameras," by J. E. D. Hurwitz, P. B. Denyer, D. J. Baxter, and G. Townsend, SPIE Vol. 3019, pages 115-124, and "Progress in CMOS Active Pixel Image Sensors," by S. K. Mendis, S. E. Kemeny, R. C. Gee, B. Pain, Q. Kim, and E. R. Fossum, SPIE Vol. 2172, pages 19-29. The image sensor typically employs an array of active pixels that are exposed to light reflected from the object or scene. Each active pixel includes photodetecting circuitry and related storage and control circuitry that converts incident light into analog electrical signals. For example, FIG. 1 illustrates in relevant part a typical prior art active pixel 104. The pixel 104 uses MOS circuit elements such as the field effect transistor (FET) and implements an electronic shutter as described below.
The following short cuts are used in this disclosure to describe various operating regions of the MOS field effect transistor (FET). An FET is said to be "turned off" when V.sub.GS (gate-source voltage).ltoreq.V.sub.T (threshold voltage) for the device and the device is operating in the cut-off region where its channel acts as an open circuit. When a FET is "turned on", V.sub.GS &gt;V.sub.dT, V.sub.DS (drain-source voltage) is normally small and the device is operating in the non-saturation region.
Turning now to FIG. 1, the prior art pixel 104 includes a photodiode D.sub.10 and a RESET transistor M.sub.13 coupled to a storage capacitor C10 via a SAMPLE transistor M.sub.14. The pixel 104 operates in response to RESET and SAMPLE signals being used to turn on M.sub.13 and M.sub.14 which causes the voltage at node A (V.sub.IN) to rise to a reset value. When the desired object or scene comes into view of the image sensor, a timer (exposure timer, not shown) is triggered and M.sub.13 is turned off. Thereafter, photo-generated electron-hole pairs in D.sub.10 cause a photocurrent (light-generated signal) Iphoto which discharges the capacitor C.sub.10 through M.sub.14 and consequently results in V.sub.IN decaying. When the timer reaches a predetermined point, M.sub.14 is turned off, leaving an exposed value for V.sub.IN on C.sub.10. M.sub.14 thus acts as an electronic shutter in limiting the light energy detected by the pixel. The difference between the reset value of V.sub.IN and the exposed value, together with the exposure or "integration" time defined by the timer, gives a measure of the incident light energy detected by the pixel.
To read the information in the reset and exposed values, the prior art pixel 104 includes a pixel output stage having M.sub.11 and M.sub.12. M.sub.11 is used as an amplifier whereas M.sub.12 is a switch. These two devices can be considered to be part of the readout circuitry in the pixel. The pixel readout circuitry together with a load (not shown) on node B form an amplifier in a source follower configuration having a voltage gain less than one but a current gain greater than one when an ADDRESS signal is applied that turns on M.sub.12. When that happens, an analog signal V.sub.OUT representative of V.sub.IN and hence the incident light energy may be read from the pixel. An example of an active pixel with associated readout circuitry is discussed in U.S. Pat. No. 5,471,515, "Active Pixel Sensor With Intra-Pixel Charge Transfer," to Fossum et al.
For a pixel having ideal read-out circuitry, V.sub.OUT will equal V.sub.IN (voltage gain of exactly one) for the entire range of V.sub.IN. However, for an actual prior art pixel such as pixel 104, V.sub.OUT is a non-linear function of V.sub.IN. Any non-linearity or deviation from the ideal presents an additional problem for the imaging system designer to deal with, as the detected image deviates from the actual scene. Such deviations are particularly undesirable in high end imaging systems such as the digital camera.
The non-linearity in V.sub.OUT is known as gain distortion and may be caused by V.sub.T modulation, where the gain of M.sub.11 in the readout circuitry is modulated in response to a changing threshold voltage V.sub.T of M.sub.11. This occurs because M.sub.11 is implemented as a n-channel FET in a P-substrate, where the P-substrate is connected to zero potential or ground. The source to substrate (bulk) voltage for M.sub.11 in this configuration is non-zero and changing for different values of V.sub.OUT. As a result, V.sub.T for M.sub.11 and therefore its gain is changing as a function of V.sub.OUT.
The prior art pixel 104 also suffers from reduced dynamic range, particularly at the low end where V.sub.IN approaches 1 volt. V.sub.OUT cannot follow such low input voltages due to the gate-source drop across M.sub.11 and the drain-source drop across M.sub.12.
In addition to gain distortion discussed above, the output signals of pixels in an imaging array are susceptible to errors (small differences between design and actual values). These may be caused by manufacturing variations among the pixels in the array and by readout noise originating in the signal path beyond the source node of M.sub.11 and magnified when referred back to node A.
To deal with such errors, a technique known as correlated double sampling (CDS) in the field of image sensor technology may be used to cancel first order errors due to device mismatches as between pixels in the manufactured sensor array and due to readout noise. Cancellation is achieved by correlating or subtracting a "dark image" output voltage (obtained for V.sub.IN being the reset value) from the "desired image" output voltage (when V.sub.IN is the exposed value) for each pixel. However, such a technique may not sufficiently correct higher order errors and errors due to gain distortion, particularly voltage-dependent gain distortion such as V.sub.T modulation.
In view of the above, it would be desirable to have a novel imaging system which may correct for some or all of the above disadvantages. Also, as the typical image sensor can employ in excess of several hundred thousand pixels, the improved imaging system should keep pixel size as physically small as possible, so as to keep the image sensor compact and permit greater image resolution through the use of a larger number of pixels.